1. Field of the Invention
The invention is generally related to the area of optical communications. In particular, the invention is related to improved designs of multi-stage optical DWDM (Dense Wavelength Division Multiplexing) channel-group interleaver to minimize signal distortion and/or transmission loss.
2. The Background of Related Art
The future communication networks demand ever increasing bandwidths and flexibility to different communication protocols. DWDM (Dense Wavelength Division Multiplexing) is one of the key technologies for such optical fiber communication networks. DWDM employs multiple wavelengths in a single fiber to transmit in parallel different communication protocols and bit rates. Transmitting several channels in a single optical fiber at different wavelengths can multi-fold expand the transmission capacity of the existing optical transmission systems, and facilitating many functions in optical networking. An international standard wavelength grid has been suggested by ITU (International Telecommunication Union) for the center wavelengths of the DWDM systems. Different technologies have been developed to divide or combine channels or subgroups of channels in the ITU grid.
It should be noted that the typical DWDM thin film technology is based on cascading single channel units into multi-channel module. Thus, when the channel counts increases, accumulated maximum insertion loss of the cascade module increases proportionally. Under this situation, the total channel count in cascading structure is normally limited below a certain number (e.g. 16). As the communication networks demand ever increasing bandwidth, the channel count in the DWDM systems increases rapidly. As mentioned in the previous paragraph, many different technologies have been developed to divide channels of the original signal spectrum into subgroups, wherein the band splitters and the interleavers are two of them.
From a terminology""s viewpoint, a device that multiplexes different wavelength channels or groups of channels into one fiber is a multiplexer, and a device that divides the multiplexed channels or groups of channels into individual or subgroups of channels is a demultiplexer. Specifically, when a multiplexer combines two streams of spectrum-periodic optical signals into a single, denser spectrum-periodic signal stream, or in reverse a demultiplexer separates a single spectrum-periodic signal stream into two complementary spectrum-periodic signal streams, such multiplexer/demultiplexer is called an interleaver. Understandably, the spectrum-periodic signal stream is defined in the optical frequency domain and the periodicity is limited within certain optical fiber transmission band. The main function of an interleaver is to separate a channel-continuous spectrum-periodic signal stream into coarser spectrum-periodic signal streams or vice versa. The typical standard interleaver is used to divide the signal spectrum into an even channel group including channels 2, 4, 6, 8 . . . and an odd channel group including channels 1, 3, 5, 7 . . . .
Somewhat differently, the band splitter is to divide the signal stream into subgroups by using the sequential thin film filters, for example, splitting the spectrum into the red and blue bands first and then splitting the spectrum further down via the same principle, for example, U.S. Pat. No. 6,067,178. Understandably, a reverse process of the band splitter may combine the spectrum signal groups.
Regarding the former, i.e., the interleaver, a variety of technologies have been exploited to achieve high performance thereof, including fused fiber Mach-Zander interferometer, multi-cavity Fabry-Perit interferometer, and polarization interference filters consisting of birefringent crystals. However, the interleaver technology faces poor thermal stability, poor isolation in non-adjacent channels, and no-flat pass band, thus requiring additional active thermal control and resulting in reliability and other concerns.
Regarding the latter, i.e., the band splitter, the problem is that the channels on the edge of the bands (edge channels) will suffer server distortion and thermal drift deterioration, and thus these affected edge channels typically are omitted when using a band splitter to divide the spectrum. Therefore, it is very hard to achieve a fully channel-continuous spectrum through the band splitter. The user has to either compromise this defective spectrum or compensate it with complicated expensive means. Anyhow, neither way is deemed economic or reliable. It can be appreciated that recently to maximize usage of the limited spectrum, the designated application channels are arranged very close to each other. It is very difficult to separate these closely arranged channels from one another by directly using the band splitter without incurring severe distortions of the channels located on the edges of the band.
Therefore, an object of the invention is to provide a device/system which can easily, economically, reliably and precisely separate the desired channels from the multiple channels under a condition that there are no longer the defective distorted channels thereof and the fully channel-continuous spectrum is achieved.
The present invention pertains to improved designs of optical DWDM channel group interleavers that use a number of layers or stages of grouped splitter filters, hence referred to as multi-stage optical DWDM channel group interleavers. According to one aspect of the present invention, an optical fiber structure for demultiplexing/multiplexing includes a coupler that splits an incoming signal with multiple channels into two substantially identical signals that are respectively and mutually exclusively processed to separate the closely located successive channels into odd and even channel subgroups for further demultiplexing or multiplexing. The spectrum characteristics to process the two identical signals are so designed that the channel distortion and transmission/insertion loss are minimized by distorting only odd or even channel groups respectively in the two substantially identical signals. In particular, multiple stages of splitter filters are used to shorten the transmission path a channel group signal has to pass through. As a result, signal distortion and transmission/insertion loss are minimized.
According to one embodiment, channel groups in an incoming optical signal are separated in a cluster fashion. Namely, each cluster includes a predetermined number of channel groups and the clusters are separated or interleaved by at least one channel group. This is achieved through an arrangement of one or more filters, the frequency response of the arrangement is so designed that the passband thereof is extended to the neighboring channels to ensure that the channels to be xe2x80x9cthroughxe2x80x9d are intact. Through repeated channel separation processes, the originally multiplexed odd and even channel groups are demultiplexed.
One of the objects in the present invention is to minimize the distortion and transmission/insertion loss incurring in guiding the optical signal through optical filters.
Other objects, features, and advantages of the present invention will become apparent upon examining the following detailed description of an embodiment thereof, taken in conjunction with the attached drawings.